The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the measurement of and subsequent compensation for non-idealities in the magnetic field gradients produced by such MRI systems.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned magnetic moment, M.sub.Z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
The application of magnetic resonance to imaging, and to many of the techniques of localized spectroscopy, depend upon the use of linear magnetic field gradients to selectively excite particular regions and to encode spatial information within the NMR signal. During the NMR experiments, magnetic field gradient waveforms with particularly chosen temporal variations are used. Any departure from the application of ideal magnetic field gradient waveforms can, therefore, be expected to introduce image distortion, intensity loss, ghosting, and other artifacts. For example, imperfect rephasing of the nuclear spins and an attendant loss of signal occurs if the magnetic field gradients are not constant during selective time reversal pulses (i.e. use of 180.degree. time reversal RF pulses). This effect compounds in later spin echoes of multi-echo (Carr-Purcell-Mieboom-Gill) sequences. In addition, if the gradient field is not zero when it should be (due to residual decay after termination of a gradient pulse), the unintended phase dispersion can result in distorted spectra in chemical shift imaging (CSI) sequences as well as inaccurate spin-spin relaxation time (T.sub.2) determination in multi-echo sequences. Those skilled in the art are thus concerned particularly about the accuracy with which time varying magnetic field gradients are produced.
Distortion in the production of magnetic field gradients can arise if the gradient fields couple to lossy structures within the polarizing magnet such as its cryostat (if the magnet is of the superconductive design), or the shim coil system, or the RF shield used to decouple the gradient coils from the RF coil. One source of gradient distortions derives from the induction of currents in these ambient structures and from the loss of energy to the shim coils. These induced currents are known as eddy currents. Due to eddy currents, one observes typically an exponential rise and decay of the magnetic field gradient during and after, respectively, the application of a trapezoid current pulse to the gradient coil.
In U.S. Pat. No. 4,698,591 entitled "A Method for Magnetic Field Gradient Eddy Current Compensation," a method is disclosed which uses an analog pre-emphasis filter in the gradient power supply to shape the current applied to the gradient coil in such a way that the eddy current induced gradient field distortions are reduced. The filter includes a number of exponential decay components and adjustable potentiometers which must be set during system calibration. A measurement technique is used prior to system calibration in which the impulse response of the uncorrected magnetic field gradient is measured and the potentiometer settings for the pre-emphasis filter are then calculated. Such techniques are described in U.S. Pat. Nos. 4,950,994; 4,698,591 and 4,591,789.
The development of faster imaging techniques such as Echo Planar Imaging (EPI), together with the development of faster gradient hardware to support such techniques, have placed greater demands on the accuracy of the generated gradient fields. This in turn has placed greater demands on the calibration methods used.
Eddy currents are described by temporal dependence and by spatial dependence (spatially invariant, spatially linear, and higher orders, e.g. quadratic). For proper calibration, acquisition of eddy current data is required for each time regime and each spatial dependence. Acquisition is followed by analysis to compute optimal pre-emphasis parameters to cancel the given temporal and spatial eddy current component.
Current calibration methods employ a fixture which supports two rf coils in the magnet bore. Simultaneous data acquisition from two coils can measure only the spatially invariant or B0 eddy currents plus the spatially linear eddy currents for one gradient axis within a single acquisition. Measurement of higher spatial orders, such as quadratic, requires an additional acquisition with at least one of the coils at a different position. Data acquisition for the other gradient axes requires repositioning the coils and performing another measurement. The current measurement fixture requires the operator to initiate data acquisition and analysis for each temporal and spatial axis separately, and to perform the analysis on each temporal and spatial axis before proceeding to the next component. Multiple iterations of data acquisition and analysis are required for each component in order to compute optimal pre-emphasis values. Results vary depending on how many iterations the operator is willing to perform or has time to perform.
Because of possible variability in operator positioning of the coils, the measurement portion of the calibration process must also include a measurement of the coil positions using an NMR experiment. Because of interactions among the various components, data acquisition and analysis for the various components must be performed in a certain order to avoid erroneous results. The entire process is very time consuming, is vulnerable to operator error because of coil positioning and dependence on the precise order of operations, and depends on operator diligence to perform as many iterations as required for optimal pre-emphasis.